Reproduction steps:
1. Open the user's attached "BugReport.zip" project
2. Open Animation window (Window > Animation > Animation)
3. Expand "Walk_Unarmed.fbx" model in Project window
4. Select "Walk_Unarmed" animation

Expected result: the animation in the Animation window is loaded in about a second
Actual result: the animation takes approximately 5-6 seconds to load in the Animation window

Reproducible with: 2018.4.34f1, 2020.2.0a16, 2020.3.4f1, 2021.1.3f1, 2021.2.0a13
Not reproducible with: 2019.4.24f1, 2020.2.0a15

Notes:
- Extra-long loading times (~10s) occur when opening the animation and the Animation window is closed. If the window is already open, on average, it takes about 5 to 6 seconds.
- If the Animation window is closed when opening the animation, the AnimationWindowState:get_allCurves() is called twice, once in AnimEditor.Initialize() and once in AnimationWindowState.OnGui(). This explains why it takes more than 10 seconds instead of the usual 5-6.